Why did Germany underestimate Soviet military reserves before Operation Barbarossa?

Germany’s failure to accurately assess Soviet military strength before Operation Barbarossa stemmed from a combination of flawed intelligence and ideological bias. German military intelligence correctly identified approximately 150 Soviet divisions stationed in the western regions of the U.S.S.R. and estimated that an additional 50 could be mobilized. However, they drastically underestimated the depth of Soviet reserves, failing to account for the more than 200 fresh divisions that Joseph Stalin was able to deploy by mid-August, which brought the total to 360 divisions.1

Furthermore, the German leadership was influenced by the mistaken political belief that the Soviet regime would collapse within three to six months due to a lack of domestic support. These miscalculations meant that although German forces successfully shattered the initial Soviet armies, they were subsequently confronted by unexpected, fresh forces that stalled their advance. This strategic error was compounded by internal disagreements among Adolf Hitler and his advisers regarding the campaign's direction following their early victories.1